1. Field of the Invention
The present invention relates to a mechanical radial scan type ultrasonic endoscope apparatus constructed in such a manner that an ultrasonic vibrating element provided in a distal end of an insertion section of the endoscope is rotated in the distal end.
2. Description of the Related Art
A mechanical radial scan type ultrasonic endoscope apparatus is disclosed, for example, in Japanese Patent Publication Kokai No. 60-227,740. In such a conventional endoscope apparatus, as shown in FIG. 1, the ultrasonic vibrating element 3 is rotatably supported at a distal end 2 of an insertion section 1 of the ultrasonic endoscope apparatus by pivoting a rotating shaft 4 of the ultrasonic vibrating element 3 on bearings 5a and 5b. The rotating shaft 4 of the ultrasonic vibrating element 3 is connected to one end of a flexible shaft 6 provided in the insertion section 1 and extending therethrough, and the other end of the flexible shaft 6 is connected to a motor 10 through a reduction gear mechanism 9 mounted in a sub-operation section 8 provided just below the operation section 7. The ultrasonic vibrating element 3 is rotated by the motor 10 through the reduction gear mechanism 9 and the flexible shaft 6, and a rotation angle of the ultrasonic vibrating element 3 is sensed by a rotary encoder 11 provided in the sub-operation section 8, so that a timing for transmitting an ultrasonic beam from the ultrasonic vibrating element 3 is controlled in accordance with a rotational deflection angle of the radial scanning line for forming an ultrasonic image.
In the conventional ultrasonic endoscope apparatus described above, since a rotation angle of the ultrasonic vibrating element 3 is sensed by the rotary encoder 11 provided in the sub-operation section 8, because of a torsion effect caused by an elasticity of the flexible shaft 6 or friction between the flexible shaft 6 and a wall surface of guide tube through which the flexible shaft 6 is passed, a drawback arises in that a phase shift occurs between the rotation angle sensed by the rotary encoder 11 and the actual rotation angle of the ultrasonic vibrating element 3 at the distal end 2. This phase shift is especially large when the flexible shaft 6 must be bent sharply to carry out the diagnosis. Therefore, in the conventional ultrasonic endoscope apparatus, since the rotation angle of the ultrasonic vibrating element 3 cannot be precisely sensed, the image for the diagnosis is formed by an ultrasonic beam directed in a different direction from the direction of the corresponding scanning line on the image displayed on the monitor, and thus the ultrasonic image is distorted and is not suitable for precise diagnosis.
In order to mitigate the above mentioned drawback, in Japanese Patent Publication Kokai Nos. 59-49,753, 60-90,542 and 60-111,642, there are described ultrasonic endoscope apparatuses in which a rotation angle detecting device is arranged in the distal end of insertion section. This device comprises one or two permanent magnets arranged to rotate together with the ultrasonic vibrating element or reflection mirror, and a magnetic sensing element such as coil, Hall element and magneto-resistive element for sensing the rotation of the permanent magnet. However, in these known devices, since the permanent magnet is used, the dimension of the distal end is liable to be large. Further, since only one or two permanent magnets are provided, the precision of detection of the rotation angle is low.
Moreover, in Japanese Patent Publication Kokai No. 58-21894, there is disclosed another known rotation angle detecting device in which a photo-resistive element is arranged in the distal end rotatably with the ultrasonic vibrating element and a beam spot is made incident upon the photo-resistive element, so that the rotation angle is derived by processing an output signal of the photo-resistive element which changes in accordance with a position on the element upon which the beam spot is made incident. In this device, since the photo-resistive element generates the output signal which is changed monotonously in accordance with the rotation, and therefore the rotation angle could not be sensed precisely.
Furthermore, in Japanese Patent Publication Kokai No. 59-67,942, there is shown a device comprising a ring-shaped rotary electrode, an array of dielectric material pits arranged circularly on the rotary electrode, and a stationary electrode over which said pits slide, so that the rotation angle is detected by sensing the change in the electrostatic capacitance across the rotary and stationary electrodes. In this known device, the rotation angle can be detected in a digital manner, but it is rather difficult to detect the change in the electrostatic capacitance in an accurate manner. Further, since the pits slide over the stationary electrode, mechanical wear is produced and the device could not be used for a long time.